1. Field of the Invention
The present invention generally relates to the use of chemical sorbents to reduce the levels of contaminants in waste streams.
2. Description of Related Art
Industrial pollutants such as heavy metals, D-block metals, mercury and arsenic pose significant health-related risks to the public. For example, several metal ions and transition metal ions have been associated with asthma symptoms such as activation of mast cells and enhanced allergen-mediated mast cell activation. Walczak-Crzewiecka, et al. “Environmentally Relevant Metal and Transition Metal Ions Enhance Fc RI-Mediated Mast Cell Activation,” Env. Health Perspectives 111(5) (May 2003). Because these substances are generated as a by-product of industrial processes, it is important to find effective means to reduce their release into the environment.
For example, mercury emissions from coal-fired utilities, commercial boilers and solid waste incinerators represent a serious environmental problem and have been the focus of many regulatory deliberations. At present, coal-fired power plants emit the largest source of mercury emissions at 32.7%. Municipal waste incinerators and non-utility boilers each contribute approximately 18% of mercury emissions. Medical waste incinerators contribute 10% of gas phase mercury emissions. In addition to gas phase mercury contamination, mercury contaminant exists in water phase as well such as water waste discharged by petroleum refineries and steel mills. For example, water phase contaminants may include elemental, ionic, organometallic, and/or inorganic mercury species.
Mercury exposure has been associated with neurological and developmental damage in humans. Developing fetuses and young children are at particular risk of the harmful effects of mercury exposure. Mercury contamination is also a concern for populations exposed to dental practices or dental waste, clinical chemistry laboratories, pathology laboratories, research laboratories, chlor-alkali facilities, and health care waste incinerators. However, despite the desire to reduce mercury emissions, presently there are no commercially available technologies to control mercury emissions.
Similarly, exposure to arsenic poses potentially significant health risks. Arsenic is a natural element, distributed throughout the soil and in many kinds of rock. Because of its ubiquitous presence, arsenic is found in minerals and ores that contain metals used for industrial processes. When these metals are mined or heated in smelters, the arsenic is released into the environment as a fine dust. Arsenic may also enter the environment from coal-fired power plants and incinerators because coal and waste products contain some arsenic. Once arsenic enters the environment, it cannot be destroyed.
Arsenic exposure causes gastrointestinal problems, such as stomach ache, nausea, vomiting, and diarrhea. Arsenic exposure can also yield decreased production of red and white blood cells, skin changes that may result in skin cancer, and irritated lungs. Inorganic arsenic has been linked to several types of cancer and is classified as a Group A, human carcinogen. In high amounts (above about 60,000 ppb in food or water), arsenic may be fatal. Similar adverse effects have been associated with other inorganic contaminants such as cadmium, chromium, lead, and selenium.
Various carbon-based sorbents have been identified for removing mercury vapor from gas streams. T. R. Carey and C. F. Richardson, “Assessing Sorbent Injection Mercury Control Effectiveness in Flue Gas Streams,” Environmental Progress 19(3):167-174 (Fall 2000). For example, Selexsorb® HG (Alcoa World Alumina, LLC, Pittsburgh, Pa.) and Mersorb® (Nucon International, Inc., Columbus, Ohio) are commercially available carbon-based mercury sorbents. Recycled tires have also been identified as a source of activated carbon that could be used for mercury removal. C. Lehmann et al., “Recycling Waste Tires for Air-Quality Control,” January 2000. Activated carbon has many drawbacks for use in large-scale industrial processes, however. In particular, commercially available activated carbon is a relatively expensive sorbent. Although transformation of waste tires into activated carbon is an environmentally friendly means of recycling harmful waste, it is a complicated, lengthy, energy-intensive and time-consuming process. Additionally, the yield of activated carbon from waste tires is relatively low.
Currently, carbon-based sorbents can be used for removal of contaminants from water, primarily through an adsorption effect of the carbon. However, this method suffers from drawbacks such as washing off of the active materials, thus making the use of carbon-based sorbent ineffective. In addition, the used active carbon materials need to be disposed of as a hazardous material, therefore adding cost and contributing to further environmental problems.
Other currently used methods include the use of catalysts to remove mercury from hydrocarbon gases. Similarly, such methods are not effective in aqueous streams due to washing off of the active catalysts.
Thus, there is a need for new technologies to efficiently and cost-effectively reduce the level of inorganic contaminants, such as mercury and arsenic for example, in industrial emissions and specifically in aqueous streams.